1. Field of the Invention
The invention relates to a fuel injection control device for a direct injection internal combustion engine in which fuel is directly injected into a combustion chamber.
2. Description of the Related Art
In a direct in-cylinder injection internal combustion engine in which fuel is directly injected into a combustion chamber, generally, fuel is pressurized by a high pressure fuel pump and supplied to a fuel injection valve. Fuel pressure is increased to a value (a target fuel pressure) at which it is possible to perform fuel injection since the fuel pressure is higher than a pressure within the combustion chamber.
In this type of fuel pressure control, a controlled variable is calculated based on a deviation between an actual fuel pressure in a fuel pipe and a target fuel pressure. The fuel pressure control is then carried out by performing feedback control of a fuel discharge amount (pump duty) of a high pressure fuel pump in accordance with the controlled variable such that the actual fuel pressure converges on the target fuel pressure. The above mentioned controlled variable used in drive control of the high pressure fuel pump is calculated using (a) an integral term that is updated in accordance with the deviation between the target fuel pressure and the actual fuel pressure, (b) a proportional term that increases and decreases in order to make the deviation between the actual fuel pressure and the target fuel pressure zero, and the like.
In feedback control of the high pressure fuel pump of this kind, when the internal combustion engine is driven at high rotational speeds, fuel pressure overshoot occurs when a cycle of a discharge stroke is shorter than a calculation cycle of the discharge amount control of the high pressure fuel pump. In one suggested method for solving this problem (refer to Japanese Patent Laid-open Publication No. 2000-282927), overshoot of the actual fuel pressure is suppressed by setting feedback gain to be smaller at high rotational speeds when the cycle of the discharge stroke of the high pressure fuel pump is short.
In fuel injection control of the direct injection internal combustion engine, if the fuel pressure is low despite a required fuel injection amount being large when the internal combustion engine starts or the like, the fuel discharge amount of the high pressure fuel pump is set to a value near to a maximum value thereof, and the fuel pressure is rapidly increased toward the target fuel pressure. At this time, even if the integral term for increasing the fuel pressure is set larger, the fuel discharge amount does not increase. As a result, the fuel pressure does not increase rapidly and the integral term is mistakenly set to an excessively high value. After the actual fuel pressure has exceeded and increased beyond the target fuel pressure, the value of the integral term starts to decrease. However, since the reduction in the value of the integral term takes place slowly, the value of the integral term is mistakenly excessively large. Accordingly, after the actual fuel pressure has reached the target fuel pressure, the controlled variable for controlling the fuel discharge amount of the high pressure fuel pump causes the fuel discharge amount to deviate to an amount that is too large as compared to the required value. Thus, overshoot occurs in which the fuel pressure exceeds and increases beyond the target fuel pressure, which may lead to problems such as deterioration in a combustion condition of the internal combustion engine.
As one approach to solving this type of problem, a method has been proposed for inhibiting the occurrence of overshoot in which updating of an integral term is prohibited when a fluid discharge amount of a high pressure fuel pump is near to the maximum value thereof (for example, Japanese Patent Laid-open Publication No. 2001-263144). As a result, it is possible to avoid the fuel discharge amount being change excessively to too large an amount due to mistaken setting of the integral term.
Further, another method has been suggested in which, as a method for maintaining favorable fuel pressure control responsiveness in feedback control of a high pressure fuel pump, when an actual fuel pressure is higher than a target fuel pressure during a fuel cut, updating of an integral term of the feedback control is prohibited (refer to, for example, Japanese Patent Laid-open Publication No. 2000-205018).
However, in the fuel injection control of the internal combustion engine, if a change amount of the target fuel pressure is changed rapidly causing the deviation between the target fuel pressure and the actual fuel pressure to transiently become larger, the actual fuel pressure is caused to track the target fuel pressure due to addition of the proportional term and the integral term in the feedback control. Even if the pump duty is calculated using the target fuel pressure and the actual fuel pressure, there is a delay before when the high pressure fuel pump is actually driven and the fuel discharged. If the timing of the next pump duty calculation occurs during this delay, the integral term increases during the delay. As a result, overshoot occurs in which the actual fuel pressure exceeds and increases beyond the target fuel pressure, whereby the combustion condition of the internal combustion engine may deteriorate.
Moreover, in the direct injection internal combustion engine, a fuel injection cycle is set to be shorter than the discharge stroke cycle of the high pressure fuel pump. Accordingly, even if the target fuel pressure is constant, if the actual fuel pressure has reduced substantially due to load variation, the load variation (the reduction in the actual fuel pressure) cannot be included in the next pump duty calculation. In this case, the deviation between the actual fuel pressure and the target fuel pressure becomes larger, and overshoot occurs in which the integral term of the feedback control increases.
The above described patent publications do not take into consideration this type of overshoot that occurs when the target fuel pressure, the load, or the like, change transiently. Further, this problem cannot be solved by methods like those disclosed in these patent publication, namely, the method in which feedback gain is set smaller at high rotational speeds, or the methods in which updating of the integral term is prohibited when (a) the fuel discharge amount of the high pressure fuel pump is near to the maximum value thereof or (b) the actual fuel pressure is higher than the target fuel pressure during a fuel cut.
Note that, in order to inhibit the pump duty from becoming less than 0% or larger than 100% in the fuel injection control of the direct injection internal combustion engine, art has been disclosed that performs processing in which the pump duty is guarded using an upper-lower limit guard (for example, Japanese Patent Laid-open Publication No. 2001-263144). With this upper-lower limit guard processing, even when the pump duty is being guarded, the integral term of the feedback control is updated. Thus, when a pump duty DT satisfies, 0%<DT<100%, overshoot of the fuel pressure may occur.